1. Field of the Invention
The present invention relates to downhole tools which can be repeatedly activated and deactivated within the well bore and, more particularly, relates to improved techniques for repeatedly setting an downhole tool suspended in a well bore from a tubular string by creating an obstruction to fluid flow through the tubular conduit to increase fluid pressure in the conduit, and for thereafter removing the obstruction to permit flow through the conduit while the downhole tool remains activated.
2. Description of the Background
Downhole tools of various types may be activated or deactivated by increasing fluid pressure in a tubular conduit in the well bore, thereby transmitting increased fluid pressure to the tool sufficient to move a piston, inflate an elastomeric member, or otherwise activate the tool. A ball or other closure member is conventionally lowered (dropped) through the conduit to cooperate with a seat and substantially restrict or terminate fluid flow through the conduit, thereby allowing for the subsequent increase in fluid pressure. This increased pressure is typically supplied by mud pumps at the surface of the well bore, such that the necessary downhole pressure required to activate the tool may be easily controlled at the surface. In most cases, the obstruction created by the ball or other closure member must be removed once the tool is activated, since other downhole equipment must frequently be passed through the conduit by a wireline or smaller diameter tubing, and/or fluid must be passed downward or upward through the tubing string.
Downhole equipment may be generally characterized as either "single-set" or "multiple-set" equipment. Single-set equipment can be activated or set in the well bore, and then be deactivated and retrieved to the surface. As the name suggests, however, single-set downhole equipment must be repaired or reworked prior to being reactivated or reset in the well bore. Multiple-set downhole equipment, on the other hand, has a capability of being repeatedly activated and deactivated in the well bore without being retrieved to the surface for repair or replacement of components. Since the expense associated with the "trip time" required to retrieve and replace a downhole tool is considerable, multiple-set downhole equipment has significant advantages over single-set tools.
One downhole tool which can be activated by increasing tubing pressure is an inflatable or hydraulically set packer. A single-set hydraulic packer assembly may typically be provided with an annular seat ring which is shear pinned to a sub which serves as a portion of the conduit which defines the tubing string. The packer may thus be set by dropping a ball to seal with this seat ring, and fluid pressure then increased in the tubing string, which is then passed to the elastomeric packer body through a flow path in the sub to inflate or set the packer in the well bore. Pressure in the tubing string may thereafter be increased beyond the packer setting pressure to shear a pin which interconnects the seat ring and the sub, thereby "blowing out" the ball. A check valve within the flow path of the sub may close off fluid flow from the packer body back to the interior of the tubing string, so that the ball removal operation does not unset the packer. Accordingly, tools and fluid may thereafter be passed through the tubing string by the location which was previously restricted by the seat and ball.
Since the ball and seat are "blown out" in a typical single-set hydraulic packer, this procedure cannot be effectively used for a multiple-set packer. While it might be theoretically possible to provide various diameter seats in a packer assembly, with each seat adapted to receive an increasingly larger diameter ball, this technique is impractical due to cost considerations and the preference for "full bore" downhole equipment. In other words, with the obstruction (ball) removed, the packer assembly preferably has a passageway substantially close to the interior diameter or bore of the tubing string, so that equipment can pass through the packer body without getting "hung up" or damaged, and so that fluid flow through the packer and thus the tubing string is not substantially restricted.
Accordingly, prior art multiple-set packer assemblies typically use a plastic material (PVC) ball to block flow through the tubing string and thereby allow for the increase in fluid pressure to set the packer. An increase in tubing pressure beyond the packer setting pressure ideally causes the ball to deform (its edges sheared), so that the ball passes through a seat greater in diameter than the normal diameter of the ball. Accordingly, a ball can be dropped for engagement with the seat, tubing pressure increased and the packer set, pressure further increased in the tubing string to deform the ball past the seat, the packer subsequently unset, and a new plastic ball dropped for repeating the operation.
A packer assembly adapted to receive a plastic ball as described above has, however, significant disadvantages. Downhole temperature is often high and variable, and temperature drastically affects the force and thus the tubing pressure required to extrude the ball past the metal seat. Since the amount of pressure required to blow the ball past the seat is highly variable, the reliability of the equipment is in question. Secondly, the outer surface of a plastic ball is frequently damaged as the ball is transported down through the conduit (dropped) to the seat. This damage to the surface of the ball thus alters the pressure required to extrude the ball through the seat and adversely affects sealing reliability with the seat. Thirdly, plastic balls generally have a density substantially close to the density of fluids which are in the conduit or tubing string. Thus, a plastic ball falls slowly through this fluid, requiring a great deal of time. Although techniques have been utilized to increase the velocity of the ball being transported through the conduit to the seat, such as providing a ball with a plastic exterior and an inner high density core, the increased velocity of the ball increases the likelihood of damage to the surface of the ball as the ball travels to the seat.
The above-described disadvantages of packer assemblies adapted to receive plastic balls have long been recognized in the art, and it is thus conventional for a multiple-set packer assembly to have one seat adapted to receive a metal ball, which seat and ball are typically "blown out" in the manner similar to that described for a single-set packer. Thereafter, plastic balls are used to repeatedly engage another "permanent" seat in the packer. The use of the metal ball thus results in high reliability for the first packer setting operation, while subsequent packer setting operations are not as reliable due to the use of plastic balls for obstructing the fluid flow through the tubing.
The disadvantages of the prior art overcome by the present invention, and improved methods and apparatus are hereinafter disclosed for repeatedly creating an obstruction in a downhole tool so that tubing pressure can be increased to activate the tool, and the obstruction thereafter easily and reliably removed to permit equipment and fluid to pass through the tubing string.